The present invention relates to digital communications, and more particularly to a method and apparatus for providing reliable decision directed adaptation (such as adaptive equalization) in systems that employ Forward Error Control (FEC).
In a digital communications system employing forward error control, the receiver is generally set to correct amplitude, phase and filtering distortions caused by the transmission channel. Such channel phenomena introduce intersymbol interference (ISI), carrier offsets and other effects that must be compensated by the receiver.
A well-known means for correcting channel distortions is to use an equalizer which performs filtering operations to remove the intersymbol interference resulting from the channel. The equalizers that are used for these applications are frequently adaptive in nature, allowing for unknown or time varying changes in the channel characteristics by adapting the equalization to compensate for the channel distortions. An equalization technique that is frequently used in communications systems is the decision-directed adaptive equalization, in which decision estimates are used by the adaptive equalizer. The decisions used by the adaptive equalizer are typically from a hard decision quantizer that does not realize the benefit of coding gain that is achieved through error control. When the pre-decoded Signal-to-Noise Ratio (SNR) is relatively low the decision and decision error values are generally unreliable. For this reason, equalization techniques using decision-directed adaptation have previously had limited success. Another approach to equalization is to use the decision and decision error values obtained from the FEC decoder, since these values are in general more accurate than the values from the quantizer. A difficulty with this approach is that the decoding delay of the FEC decoder interferes with an effective and robust method of adaptation. The introduction of any substantial delay in an adaptation loop renders such methods ineffective.
To compensate for other channel effects such as carrier offsets, a method known as a decision directed phase-locked-loop (PLL) is often employed. Again, the relatively low SNR before the FEC decoder and the delay of the decoded data coming from the output of the decoder lead to problems in updating the carrier tracking method.
For the foregoing reasons, there is a need for a method and apparatus for reliable decision-directed adaptation that takes direct advantage of the error detection of the FEC components of a system. In particular, it would be advantageous to provide such a method and apparatus that directs the updating of an adaptive element using a decision-directed adaptation scheme, and that provides for the reliable updating of the coefficients of the adaptive elements.